The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus including a movable belt and a cleaning blade for removing toner deposited on the belt. More particularly, the present invention is concerned with an image forming apparatus of the type including an image transfer/sheet transport device having a belt capable of supporting and conveying a sheet or recording medium in order to transfer a toner image from an image carrier to the sheet.
An electrophotographic image forming apparatus includes a photoconductive element. After the photoconductive element has been uniformly charged, it is directly exposed imagewise by an exposing device or is scanned by, e.g., laser optics or an LED (Light Emitting Diode) array in accordance with an image signal. As a result, a latent image is electrostatically formed on the drum. The latent image is developed by toner stored in a developing device to turn out a toner image. The toner image is transferred to a paper sheet, film or similar recording medium either directly or via an intermediate transfer body. The recording medium with the toner image is conveyed to a fixing device in order to fix the toner image thereon.
In an apparatus of the type described, a photoconductive belt or image carrier or an intermediate transfer belt is a typical belt capable of moving with toner deposited thereon. A belt included in an image transfer/sheet transport device for conveying a recording medium in order to transfer a toner image from an image carrier to the medium is another conventional movable belt. Usually, a cleaning blade is used to remove toner undesirably deposited on the surface of any one of such movable belts.
The belt included in the image transfer/sheet transport device is an endless belt formed of rubber or similar elastic material and having a medium resistance. The belt is passed over a drive roller and a driven roller and located to face a photoconductive element or image carrier or an intermediate transfer body. At the time of image transfer, the belt is brought into contact with the image carrier and conveys a recording medium by nipping it in cooperation with the image carrier. A high voltage for image transfer is applied to a bias roller, bias brush or similar bias applying means facing the rear of the belt. As a result, a toner image is transferred from the image carrier to the recording medium. The recording medium with the toner image is separated from the image carrier and then conveyed toward a fixing unit.
Assume that the apparatus including any one of the photoconductive belt, intermediate transfer belt and belt built in the image transfer/sheet transport device is operated to produce a number of copies each having a small image area or accidentally operated to produce a number of white copies. Then, no toner is left on the belt, and therefore no toner exists at the edge of the cleaning blade. In this condition, the coefficient of friction .mu. between the belt and the cleaning blade increases, increasing the frictional resistance. It is therefore likely that the edge of the blade is caught and turned over by the surface of the belt. The blade so turned over brings about defective cleaning which would smear the rear of sheets and would cause the toner to fly about, and damages the surface of the belt. This is particularly true with the belt of the image transfer/sheet transport device because only a small amount of toner reaches the blade, compared to the other belts of the kind directly carrying a toner image thereon.
To solve the above problem, when the image carrier is implemented as a belt, a toner image representative of an exclusive line for toner feed may be formed on the image carrier between sheets so as to feed toner to the edge of the cleaning blade assigned to the image carrier. In the case of the belt of the image transfer/sheet transport device, the toner image representative of the exclusive line may be formed on the image carrier between sheets and then transferred to the belt so as to feed toner to the edge of the cleaning blade assigned to the belt. The toner fed to the edge of any one of the cleaning blades prevents the coefficient of friction between the belt and the blade and therefore the frictional resistance from increasing due to the absence of toner. This successfully prevents the edge of the blade from being turned over.
In the case of the belt of the image transfer/sheet transport device, the line for toner feed is formed, e.g., once for a plurality of copies. The line is written over the entire image width of the photoconductive belt or similar image carrier between sheets, and then transferred to the belt. The edge of the blade is prevented from being turned over more positively as the line is formed more frequently. This, however, aggravates toner consumption or brings about excessive toner feed which would result in defective cleaning.
The line for toner feed will be needless if toner contaminating the background of the image carrier is transferred to the belt of the image transfer/sheet transport device and brought to the edge of the associated cleaning blade in a sufficient amount. In practice, however, the amount of toner contaminating the background is noticeably dependent on humidity; it is almost zero at the humidity of 90%. The line is therefore essential, considering the operation of the apparatus in a humid environment. While the contact pressure of the cleaning belt acting on the belt, among others, may be increased in order to ensure the removal of the line, this kind of scheme enhances the cleaning ability to an excessive degree during usual cleaning and causes the blade to deteriorate rapidly.
Some different schemes have been proposed to achieve an improved cleaning ability with a relatively low contact pressure of a cleaning blade in relation to a cleaning device assigned to a photoconductive belt. Japanese Patent Laid-Open Publication No. 60-107686, for example, teaches a cleaning blade held in contact with a photoconductive belt in an inclined position relative to the direction in which the belt runs. Japanese Utility Model Laid-Open Publication Nos. 3-45572 and 3-47574 each discloses a straight cleaning blade held in contact with a cylindrical photoconductive element in an inclined position. With any one of these conventional schemes, it is possible to prevent a great amount of toner from reaching the cleaning blade at the same time and causing defective cleaning to occur. Therefore, any one of such schemes is applicable to a cleaning device assigned to a movable belt in order to remove the toner of the exclusive line under usual cleaning conditions. However, the inclined blade occupies a broader space and must contact the flat portion of the surface of the belt. Should the inclined blade contact, e.g., the curved portion of the belt passed over a roller, the pressure of the blade would not act evenly, i.e., it would sequentially decrease toward the opposite ends of the blade and would thereby bring about defective cleaning.